1. Field of the Invention
The present invention particularly relates to stretch-blow molding process and apparatus suitable for use in forming a wide-mouthed container with its lip portion having a relatively wide opening area.
2. Description of the Related Art
The conventional process of stretch-blow molding general hollow containers including wide-mouthed containers typically comprises the steps of:
(1) injection molding a preform form a thermoplastic material;
(2) regulating the temperature of the injection molded preform into an optimum level for stretch-blow molding; and
(3) performing a biaxial orientation by stretching the inner bottom wall of the preform in its axial direction while maintaining the optimum temperature therein and substantially at the same time blowing a pressurized fluid into the interior of said preform to stretch the wall of the preform in its radial direction. In certain cases, the second step aforementioned may be omitted.
FIGS. 6 and 7 illustrate such a convention stretch-blow molding process. A preform 10 is placed in a blow cavity mold 16 with the outer periphery of its lip portion being fixedly supported by a lip mold 12. The inner periphery of the lip preform portion receives a blow core mold 14. The preform 10 is stretch-blow molded into the blow cavity mold 16 against its inner cavity wall to form a finished product with an outline following the inner wall of the cavity.
A vertically movable stretching rod 18 is supported by the blow core mold 14, with the lower end thereof having an tip piece 20. The tip piece 20 is adapted to engage the inner bottom wall of the preform 10. FIG. 6 shows that the tip piece 20 has a relatively small surface area while FIG. 7 shows that the tip piece 20 has a relatively large surface area.
When the stretching rod 18 is downwardly moved, the tip piece 20 is also moved to urge the bottom wall of the preform 10 in the downward direction such that the preform 10 will be stretched in the axial direction of the rod.
The blow core mold 14 includes a blow air introducing port 22 formed therein through its top wall and extending about the stretching rod 18. Usually, a compressed fluid (e.g. air) is blown into the interior of the preform 10 through the port 22 substantially at the same time when the stretching rod 18 is moved.
However, such an arrangement has the following problem during the stretching operation.
First, with the stretching rod 18 having a relatively small tip piece 20 as shown in FIG. 6, the downward movement of the stretching rod 18 sufficiently stretches portions of the preform 10 at and near a region engaged by the tip piece 20 of the rod 18 to make these portions thinner (see FIG. 8(A)). A product 24 stretch-blow molded in the biaxial orientation will include its thinner bottom and adjacent walls and thicker wall portions 24a near the lip portion of the container (see FIG. 8(B)).
On the contrary, with the arrangement as shown in FIG. 7, the tip piece 20 has a relatively large surface area. When the stretching rod 18 is moved downwardly, thus, that portion 10b of the preform 10 engaged by the tip piece 20 will not be sufficiently stretched by the tip piece 20 and remain as it is thicker (see FIG. 9(A)). After such a preform 10 has been stretch-blow molded in the biaxial orientation, thus, the bottom and adjacent walls remain thicker as shown by 26a while the wall portions near the lip portion are reduced in thickness (see FIG. 9(B)).
In accordance with the prior art, the surface area of the tip piece 20 of the stretching rod 18 adapted to engage the inner bottom wall of the preform is avoidably invariable resulting in not-uniform wall thickness throughout the molded container.